The invention relates to trays for packaging meat, produce and the like. Typically, the product is supported on the tray and the whole is enclosed as by wrapping in a transparent plastic film. In order that the overwrap shall be closely applied for conservation of space, for maximum exclusion of air and other purposes, the preferred wrap is shrink film which contacts on heating to a suitable temperature or stretch film which is stressed during application and contracts after application by reason of its elasticity.
Trays presently in common use for the purposes started are constituted by a flat bottom, usually rectangular in outline, surrounded by side walls which typically flare upwardly and outwardly. To a considerable extent, the outward flare of the side walls is dictated by manufacturing, transportation and storage considerations. Because the trays are intended for discard after a single use, they must be of low cost, a criterion satisfied by high-speed molding on automatic machinery. Plastic foams and clear plastic sheet are thermoformed to the desired shape by heating a web of the plastic to a temperature suitable for draw molding and impressing the heated plastic between cooperating dies which draw the plastic to the form determined by the space between the dies. Like forms of pulp are prepared by feeding a slurry of cellulose fibers to a screen of the shape desired and drawing suction on the side of the screen opposite the slurry feed to felt the fibers on the screen and dewater the product. Upon drying, this technique results in a tray of which one side bears imprint of the screen and the other is a rough surface characteristic of random fiber deposition.
Each of these methods of forming by automatic machinery forbids undercuts or reentry surfaces which inhibit rapid release from the forming equipment. Indeed, surfaces normal to the bottom of the tray are avoided because the frictional drag on disengagement from forming surfaces can seriously impede smooth operation and promote jamming of the equipment due to improper orientation of articles which release from molds more easily at one portion than at other portions of the article.
Further constraints to similar effect are imposed by mandates of transport, storage and use. The trays for merchandizing of meat, produce and other products are of relatively low density, i.e. weight per unit of volume. Costs for transport and storage are therefore based primarily on the space occupied by a quantity of the articles. Volume occupied by a quantity of trays is reduced to the extent the articles nest, one within the next in a stack, with minimum space between them. That desirable result is served by outward flaring of side walls.
For ease of molding and to facilitate nesting, sharp angle corners are to be avoided, thus sacrificing the strengths inherent in webs which meet at well defined angles. Since the materials of construction are of relatively low rigidity and compressive strength, it becomes desirable to add structural strength by such devices as a flange extending outwardly of the top edge of the side wall.
All such expedients suffer the disadvantage that misalignment of the trim tool which removes excess material from margins to prepare the finished article can cause one or more edges to be unduly weakened. Such misalignments are not unusual in commercial operation and frequently result in inferior products, at times so serious as to require condenmation of finished product to scrap for reworking.
Inferior products from slight misalignment of trim tools aggravate a problem in packing of food products which is encountered even with perfectly formed and trimmed trays. These can be loaded and wrapped by hand operators without excessive difficulty, if the operator is attentive, but a different situation arises where, e.g. California, meat cuts and the like are packed in trays and wrapped by machine at a central butcher warehouse for delivery to and sale from marketing outlets remote from the point of packing. The packing machines in such installations include a ram which projects the loaded tray at high speed and positive thrust against a web of shrink film or stretch film, draws the film under the tray and seals it, followed by radiant heating in the case of shrink film. Depending on the angle at which the web of film encounters the edge of the tray wall, the latter may be bent downward, bent inward, or subjected to a compressive strain; resulting in unsightly distortion of the tray or actual fracture of the side wall with consequent risk of seepage of meat juices or the like into the wrapping under the package. As noted above, these problems are severely aggravated when misalignment of the trim tool causes production of packaging trays which vary, even slightly, from the design for which the central butchery operation has caused its packaging machines to be set.